The ablative marking of products with laser energy has become widespread for a number of reasons. The laser marking process is very fast, features ultra high resolution, can be reprogrammed on the fly with only a software change, requires no consumables such as ink, does not smudge as does wet paint, and requires no drying time.
Because of the aforementioned advantages over other types of marking processes, laser marking is used extensively for the marking of semiconductor packages. Although initially utilized for placing coded identification relating to manufacturing (e.g., lot number) on the bottom of semiconductor packages, lasers are now being used with increasing frequency to engrave the manufacturer's logo and part number on the top of packages.
One of the problems associated with any marking process, whether laser or offset-ink based, is the problem of intermittent equipment malfunction, which may result in unmarked items. Manual inspection of items for the purpose of identifying those which have not been marked is both costly and somewhat ineffective, as only about 80% of the unmarked items are typically discovered. Rework of the unmarked items can also be costly and inefficient. What is needed for the laser marking process is a method and apparatus for detecting when a part is not marked during the marking operation itself.